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Direct Return Pump Question

Good evening. Pondering some would be circumstances on a job I looked at. From the look of the piping it looks to be direct return. Boiler like most is over sized. Piping over sized. Radiators much larger than needed. Assuming it is a direct return system at this point. With three zones, each for its own floor, cast iron wall radiators all around the home. Calculating the DT and reverse doing the math to plot it on the current pump, I see that the Bumble Bee will fit the bill. Staying away from calculating using worst case scenario length x .06 because the pump will be grossly over sized in comparison to what is there and has been working for 15 years.

My question is, when the pump slows down to widen the dt, will the water short circuit through the closest radiators due to the lack of constant head pressure? Adding trvs under each radiator is out of the question.

Im am not concerned about the upper floors as the thermostat will close upon heat need being satisfied. The boiler output temp will be sized close to the needed temp to run continously so less of a problem.

Thinking of trying the new Cadet boiler by lochinvar.

As for zone valves, other than White Rodgers, who makes 1 1/4 inch or larger valves with a high CV (low pressure drop)? Tavo has high cv valves only in 1 inch and below.